7: Low Methanol Permeable Nafion-Organically Modified
7.3 Results and Discussions
7.3.1 Morphology
Figure 7.1 shows the high-resolution optical micrographs of Nafion nanocomposite films with different C30B contents. The highly anisotropic black spots in Figure 7.1 a-c are attributed to MMT particles dispersed in Nafion. The micrographs show the even distribution of C30B when 0.5 and 2 wt% were incorporated. This dispersion impacts anisotropic properties to the polymer matrix, which should improve barrier properties. This suggests that the hydroxyl groups available in the interlamellar space of C30B interact favorably with Nafion. This contributes to the polymer penetrating the clay galleries and the delaminating of the alumino-silicate stacks by the shearing forces during melt blending. However, large alumino-silicate stacks are observed with the 1 wt% C30B-containing nanocomposites. This suggests that polymer chains could not penetrate the clay galleries properly.
Figure 7.1 Optical miscropcopy images of Nafion nanocomposite films with different C30B loadings: (a) 0.5 wt%, (b) 1.0 wt%, and (c) 2.0 wt%.
7.3.2 FT-IR Spectral Analyses
FT-IR spectra shown in Figure 7.2 of C30B showed absorption bands at 1065 cm–1 and 520 cm–
1. These are attributed to water in the interlays, hydroxyl bending of adsorbed water, and bending of Si–O–Si and Si–O–Al. Nafion showed adsorption bands at 600, 950, 1150–1200 cm–1 and these are ascribed to CF–CF2 and SO2–O stretching, respectively.[15–16] FTIR spectra showed that the incorporation of C30B, in different proportions into Nafion leads to a decrease in the intensity of the bands (OCF–CF2 and CF2–S2–O). This suggests that side chains of Nafion resin may be involved in the interactions with C30B. However, the position of Nafion bands remains unchanged. This indicates that the molecular motions of the OCF2 and SCF2 are considerably restricted, probably due to the interactions with C30B. This is consistent with previous observations.[17–19]
(c)
100 µm
(a)
100 µm
(b)
100 µm
800 1200 1600 2000 Nafion C30B
N-0.5%C30B N-1.0%C30B N-2.0%C30B
Intensity / (a.u.)
Wavenumber / (cm-1)
Figure 7.2 Fourier transform Infrared spectra of Nafion, C30B and Nafion nanocomposite films with different loading of C30B.
7.3.3 XRD Patterns
Figure 7.3 shows the XRD patterns of the Nafion resin and corresponding nanocomposites containing various contents of C30B in the 2θ range 5–70°. The membranes show the main peak at 18°, and this is related to the hexagonal structure of Nafion. The broad peak around 40° is associated with the fluorocarbon chains of Nafion, and it highlights the poor crystallinity of Nafion matrix.[20–22] The d-spacing values of Nafion and its nanocomposite membranes are calculated and summarized on Table 7.2. The average interlayer spacing for Nafion, 0.5 wt%
and 1 wt% nanocomposites obtained from XRD measurements is 0.5 nm (2θ = 16°). The peaks shifted towards higher angles after incorporation of C30B. This suggests a decrease in d-spacing
0 100 200 300 400 500
10 20 30 40 50 60
Nafion
N-0.5% C30B N-1.0% C30B N-2.0% C30B
Intensity/ (a.u.)
2theta/ (deg.)
with an increase in C30B content. The change in the crystallinity or crystal structure may improve the properties of Nafion.
Figure 7.3 X-ray diffraction patterns of Nafion and its C30B composites with different C30B loadings: 0.5wt%, 1 wt%, and 2 wt%.
Table 7.2 Interlayer distances of Nafion and various Nafion-C30B nanocomposite membranes.
Samples Interlayer distance d /nm
Nafion 0.50
N-0.5 wt% C30B 0.48
N-1.0 wt% C30B 0.48
N-2.0 wt% C30B 0.55
0 20 40 60 80 100
0 100 200 300 400 500 600 700 Nafion
N-0.5%C30B N-1%C30B N-2%C30B
Remaining mass / (wt.%)
Temperature/ (deg.C)
7.3.4 Thermal Stability
The thermal stability of Nafion resin and its C30B-containing nanocomposite films was investigated by TGA. Nafion showed weight loss of about 5% between 25–150°C whereas the Nafion-C30B nanocomposite showed about 2.5% weight loss. This weight loss is due to the loss of physically adsorbed water. Between 150–350°C, Nafion showed a weight loss of 5% whereas N-C30B nanocomposite showed weight loss of about 7.5% (see Fig. 7.4). This is attributed to the loss of SO2 and CO2 gases as a result of Nafion decomposition.[23] The last degradation stage between 350–500°C is attributed to the further loss of SO2, CO2, and the release of SF4, CO, CF, and CF2 from the backbone of Nafion.[24] It is observed that the weight loss associated with backbone of Nafion is moderate for nanocomposite compared to Nafion resin.
Figure 7.4 Thermogravimetric analysis scans of Nafion and its C30B-based composites with different C30B loadings: 0.5 wt%, 1 wt%, and 2 wt%, measured in air at 5°C/min.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0 50 100 150
Nafion N-0.5%C30B N-1%C30B N-2%C30B
Tan delta
Temperature /( deg.C)
7.3.5 Mechanical Properties
Figure 7.5 The temperature dependence of tan delta (tan ) of neat Nafion and its C30B- containing nanocomposite membranes with different C30B loading.
DMA results in Figure 7.5 show the temperature dependence of tan δ curve of the neat Nafion resin and C30B-containing composite membranes, with different C30B loadings. There are two peaks observed in the temperature range examined from –30 to 180°C. The first peak (30°C) is attributed to the transitions between SO2 and CF groups of Nafion. The second peak at about 140°C is assigned to the α-relaxation, which is close to the glass transition temperature (Tg) of the ionic clusters of Nafion resin. [24] The higher the Tg of the material, the better their thermo- mechanical stability. For neat Nafion membrane the maximum tan δ or Tg peak appears at about 140°C. However, when C30B was incorporated into the Nafion matrix, the Tg of Nafion matrix shifts to the higher temperature range. The highest Tg of about 150°C is observed in the case of N-2 wt% C30B composite membrane. This confirms that N-2 wt% C30B composite membrane
0 1 2 0.0
3.0x10-7 6.0x10-7 9.0x10-7 1.2x10-6
Dc-conductivity Water-uptake
C30B Loading/(wt%) Dc-conductivity/ (S cm-1 )
12 14 16 18 20
Water-uptake/ (%)
has an excellent thermo-mechanical stability compared to other membranes. This dramatic improvement in mechanical stability is due to the good dispersion of 2 wt% C30B into Nafion matrix observed by optical microscopy. The results showed that as the amount of C30B is increased within Nafion, Tg also increases. This indicates an improvement in mechanical stability. This suggests a possible strong interaction between the ammonium salt used to modify MMT and sulphonic groups of Nafion matrix.
7.3.6 Electrical Conductivity and Water Uptake of the Membranes
Figure 7.6 Electrical conductivity and water uptake measurements of Nafion and nanocomposite membranes with different C30B loadings.
Figure 7.6 shows the relationship between both water uptake and electrical conductivity versus the weight fraction of C30B in the nanocomposite membranes. The water uptake of N-0.5 wt%
C30B and N-1 wt% C30B nanocomposite membranes decrease to about 12% compared to 20%
of neat Nafion. However, water uptake of N-2 wt% C30B slightly increases to about 18%. This might be due to the surface modification of MMT with a quaternary ammonium salt. [25–26] These results suggest that water uptake is dependent on the amount of C30B dispersed in Nafion. As
shown in Figure 7.6, the bulk electrical conductivity decreased to zero after incorporation of C30B in Nafion. The electrical conductivity remains zero independent of the C30B amount. This is a good beneficial effect for the fuel cell application as short-circuiting will not be possible and such results have never been reported before. Nanocomposite preparation methods and dispersions play a huge role in these observations.
7.3.7 Proton Conductivity and Methanol Permeability
Table 7.3 shows the methanol permeability and proton conductivity of neat Nafion and Nafion nanocomposite membranes with different C30B loadings. The methanol permeability of neat Nafion is higher, with a value of 2 x 10–8 cm2.s–1, than that of Nafion-C30B nanocomposite membranes (1 x 10–8 cm2.s–1). This suggests that methanol permeability can be controlled by the addition of C30B to Nafion. On the other hand, proton conductivity decreased with the incorporation of C30B. This is associated with the lower water uptake of the nanocomposite membranes. This suggests that C30B modified membranes allow selective permeability of water molecules or hydrogen ions. The lowest decrease is observed with N-0.5 wt% C30B. This might be caused by the poor cohesion of 0.5 wt% C30B into Nafion.
Table 7.3 Comparison of proton conductivity (C) and methanol permeability (P) of neat Nafion resin and Nafion-C30B nanocomposite membranes, with different C30B wt% loadings measured at 25°C and 100% humidity.
Samples C /S.cm-1 P /cm2.s–1 C/P /S.cm–3
Nafion 70 x10–6 2x10–8 4x103
0.5 C30B 4.5 x10–6 1x10–8 5x102
1.0 C30B 15 x10–6 1x10–8 2x103
2.0 C30B 30 x10–6 1x10–8 3x103
It can be seen from Table 7.3 that a decrease in proton conductivity is minor compared to a decrease in methanol permeability. The methanol permeability of the nanocomposites is independent of the amount of C30B. However, the proton conductivity is affected by the amount of C30B incorporated into Nafion. As the amount of C30B in Nafion increases, the proton conductivity slightly increases. This also depends on the dispersion of C30B in Nafion matrix.